Magneto-optic storage systems using the Faraday effect of a magnetizable thin film in combination with a polarizer/analyzer pair is well known in the art--see the R. C. Sherwood, et al, U.S. Pat. No. 3,059,538. In the publication "An Overview of Optical Data Storage," Di Chen, et al, Proceedings of the IEEE, Volume 63, No. 8, August, 1975, Pages 1207 through 1230, there is presented a review of various techniques for storing data information in optical storage devices. With particular reference to the present invention, the two techniques discussed therein are Curie point writing and compensation temperature writing. Curie point writing is a method in which the temperature rise in the heated spot of the magneto-optic storage device exceeds the Curie temperature of the storage medium. During cooling from the Curie temperature, the magnetic closure flux and the applied external field can effectively determine the direction of magnetization of the heated bit. Thin films of MnBi at room temperature and EuO at cryogenic temperatures have been most extensively studied along with many other materials proposed for the Curie point writing techniques.
Compensation temperature writing is a method using certain ferrimagnetic materials such as gadolinium iron garnet (GdIG), with two sublattice magnetizations in opposite directions. At the compensation temperature of the storage medium, these sublattice magnetizations cancel out each other and the storage medium attains extremely high coercivity H.sub.c. A few degrees away from this compensation temperature the coercivity H.sub.c drops and magnetization switching becomes easy. By operating the storage medium at the compensation temperature, a switching field is applied in coincidence with a laser heating pulse, which allows the heated spot to rise above a temperature at which the coercivity is below the applied switching field. This method of writing has been experimentally demonstrated in single crystals and thin films of GdIG. A variation of this technique is based on the use of compensation wall domains in Ga-substituted YIG. Additionally, the use of the Faraday rotation of a plane polarized light beam that is incident to an iron garnet film has been studied for some years--see the publication "Faraday Rotation In Rare-Earth Iron Garnets," W. A. Crossley, et al, Journal of Applied Physics, Volume 40, No. 3, March 1, 1969, Pages 1497-1498.
Optical systems for the display of multi-colored displays have, in the past, utilized many techniques. Such systems have included systems for selectively orienting the crystal axis of a piezoelectric light valve to control the transmission of light of various wavelengths to achieve multi-colored displays--see the H. Jaffee U.S. Pat. No. 2,616,962, an electro-optic crystal controlled variable color modulator for producing multi-color images in TV receivers--see the T. F. Hanlon U.S. Pat. No. 3,428,743, and ferro-electric ceramic wafers for the switching of the three basic colors above the flicker fusion frequency in TV receivers--see the M. N. Ernstoff, et al, U.S. Pat. No. 3,783,184. Additionally, in the publication "3-Color Laser Beam and Acousto-Optic Cell Pave the Way for Practical Uses," E. Dilatush, EDN, July 5, 1974, Pages 16, 17, there is discussed an optical system wherein a single laser, which emits light of red, green and blue wave lengths, is fed through an acoustic-optic cell that is simultaneously driven by three modulating frequencies. By adjusting the three modulating frequencies, three of the beams, one of each color, can be made to come out of the acoustic-optic cell at the same angle. The final result is a single modulated three-colored collinear beam that can be deflected and, in turn, projected upon a display screen. The present invention is considered to be an improvement over these other known optical systems.